(CNN) Sen. Elizabeth Warren this week fired a clear warning shot at billionaire Democrats considering running for president by calling for a rejection of super PACs and self-funding in the 2020 primary.
Warren's comments, which first came at a New Year's Eve news conference after she announced her 2020 exploratory committee and were followed up in an interview with MSNBC on Wednesday, made clear one of the first litmus tests of the 2020 Democratic primary and were quickly echoed by a progressive organization backing Warren.
The large field of potential Democratic candidates includes two prominent billionaires, former New York Mayor Michael Bloomberg and investor Tom Steyer. Both demonstrated in 2018 that they are willing to use their considerable resources for political purposes, spending over $100 million each on state and federal elections during the midterm elections.
"I think this is a moment for all of the Democratic candidates as they come into the race to say: In a Democratic primary, we are going to link arms and we're going to grass-roots funding. No to the billionaires," Warren said on MSNBC's "The Rachel Maddow Show."
She added: "No to the billionaires whether they are self-funding or whether they're funding PACs. We are the Democratic Party, and that is the party of the people. That's how we not only win elections, that's how we build movements that make real change."
When pressed specifically on whether Bloomberg and Steyer should be barred from running, Warren said, "Of course not. I just mean people should not be self-funding. And they should not be funded from PACs from other billionaires. The primary is an opportunity to hear from the grass roots, to see what you can build."
Steyer spokeswoman Aleigha Cavalier declined to directly challenge Warren's comments but did say the lead-up to 2020 is a time for Democrats to "come together," not divide each other.
"Americans are looking for leaders who have a positive vision of the future and who are willing to tell the truth on issues like climate change, impeachment and justice," Cavalier said. "Now isn't the time to be divided -- it's time for Democrats to come together and figure out how we can head into 2020 as strong as possible."
A Bloomberg spokesman did not respond to a request for comment.
The warning builds on Warren's anti-super-PAC messaging. The senator wound down PAC for a Level Playing Field, her leadership political action committee, near the end of her successful 2018 re-election campaign, spokeswoman Kristen Orthman told CNN, and is in the process of winding down the Elizabeth Warren Action Fund, her joint fundraising committee.
Warren's call to reject PACs and self-funders was quickly echoed by progressive organizations like the Progressive Change Campaign Committee.
"This should be an easy one. Many winning House candidates rejected corporate PAC money in 2018 -- and won," the group, which supports Warren, said in an email to supporters. "All Democratic presidential candidates should reject and actively discourage billionaires from funding shady efforts on their behalf."